The nucleus locus coeruleus (LC) has been implicated in both the physical signs and adverse withdrawal effects of opiates. The physiological basis postulated for these actions includes opioid activation of receptive sites of noradrenergic neurons and may also involve interactions with serotonin and glutamate. The proposed studies will examine the cellular mechanisms underlying these important physiological interactions. The methods include dual labeling electron microscopic immunocytochemistry and combined immunoelectron microscopy and in vivo intracellular physiology in adult rat brain. Aim I will examine the cellular basis for modulation of noradrenergic neurons by enkephalin opioid peptides. Potential sites of interaction to be examined include (1) direct synaptic input to catecholamine neurons by enkephalin terminals (2) common targets for enkephalin and/or catecholamine containing terminals. Additionally, electron microscopic immunocytochemistry combined with tract-tracing methods will be used to determine whether opioid terminals synapse onto LC neurons projecting to the frontal cortex. Aim II will determine the cellular substrates for (1) modulation of catecholamine-containing neurons by serotonin and glutamate afferent terminals and (2) proposed interactions between these terminals and terminals containing opioid peptides in the LC. Aim III will test the hypotheses that frontal cortical neurons recorded intracellularly in vivo respond to LC stimulation and receive direct synaptic input from noradrenergic terminals. This multidisciplinary anatomical and physiological approach will provide basic science information relative to understanding how diverse afferents (opioid, serotonin, glutamate, catecholamine) modulate noradrenergic neurons in the LC in the rat. The findings may have direct applicability to further understanding the synaptic circuitry underlying many cognitive and analgesic effects of opiates in humans.